The present invention relates to a charged beam exposure method and charged beam exposure apparatus, and, more particularly, to a charged beam exposure method and charged beam exposure apparatus for use in aligned exposure.
A typical alignment method to be used in exposure using a charged beam, for example, an electron beam is to scan a mark formed on the base substrate with a fast-accelerated electron beam and detect back-scattered electrons or secondary electrons from the mark. This mark for alignment is formed in a step structure or formed of materials with different back-scattered electron emission efficiencies.
Recently, studies are being made on exposure with a low-energy electron beam of 1 to 2 kV because it provides enhanced resist sensitivity and lower proximity effect and is difficult to cause charge-up (see, for example, J. Vac. Sci. Technol. 10 (1992) 2743, xe2x80x9cArrayed miniature electron beam columns for high throughput sub-100 nm lithographyxe2x80x9d by H. P. Chang et al.). Such a low-energy electron beam however has a poor penetration capability.
The problem that arises from mark detection using a low-energy electron beam will be discussed below referring to FIG. 1. As shown in FIG. 1, a sample has a lamination structure of an Si substrate 21, an Si oxide film 22 and a resist 24. An alignment mark 12 is formed on the Si substrate 21 deep from the surface of the resist 24. When a low-energy electron beam 10 is used, the electron beam 10 does not reach the alignment mark 12 formed deep from the surface of the resist 24, so that a back-scattered electron signal cannot be acquired. While there has been proposed provision of separate electron beam sources respectively for mark detection and exposure (e.g., Jpn. Pat. Appln. KOKAI Publication No. Hei 2-37710, Jpn. Pat. Appln. KOKAI Publication No. Hei 7-169665 and Jpn. Pat. Appln. KOKAI Publication No. Sho 63-263720), this approach makes the exposure apparatus complex.
As a solution to the problem that a back-scattered electron signal cannot be acquired, a mark detection method as illustrated in FIG. 2 is proposed. As shown in FIG. 2, this method scans only a resist surface 182 with the electron beam 10 to charge up the resist surface 182 so that the image of the alignment mark 12 is seen as secondary electrons 183.
This method utilizes the fact that there is a contrast between a capacitance 184 applied to the insulating film above the mark 12 differs from that in the other region, which makes the secondary electron emission efficiency directly above the mark 12 different from that on the other region. FIGS. 3A and 3B show the pattern image and waveform of the alignment mark 12 detected by this method. FIG. 3A is a top view of the pattern image of the alignment mark 12, and FIG. 3B shows the detected waveform of the pattern image. Although this method has been discussed with reference to an example wherein the resist film is used as the insulating film, an Si oxide film or the like may be used as the insulating film without changing the effect.
This mark detection method however has the following shortcomings. In a case of performing electron beam exposure on an insulating film like a resist, charge-up may cause misalignment of the beam. Conventionally, to eliminate this charge-up, a conductive film 201 was formed under (or on) the resist 24 as shown in FIG. 4. Here, the conductive film 201, like the Si substrate 21, is grounded to an earth 7. In such a case, no charge-up occurs, thus ensuring electron beam exposure without misalignment. Because the whole conductive film 201 has the same potential, however, there is no capacitance contrast between a region above the alignment mark 12 and the other region. This makes it difficult to detect the image of the alignment mark 12.
Further, inclination of a sample to the objective lens lowers the precision of alignment. FIG. 5 is a diagram for explaining this problem. FIG. 5 shows a case where a sample 2 is tilted with respect to, for example, an objective lens 211 at the time a voltage is applied to the sample 2. In this case, an electric field 212 between the sample 2 and the objective lens 211 become non-uniform, changing the traveling path of the electron beam 10.
As apparent from the above, the conventional exposure method using a low-energy electron beam suffers a low penetration capability of the electron beam, which makes it difficult to ensure high-precision alignment exposure with a simple system structure. With regard to the mark detection scheme which utilizes the capacitance contrast, when a conductive film is formed on or under the resist to prevent charge-up of the resist, the alignment mark cannot be detected. When the traveling direction of the electron beam deviates from the inclination of the surface of the sample, it is difficult to perform high-precision alignment.
Accordingly, it is an object of the present invention to provide a charged beam exposure method which can accomplish high-precision alignment and pattern exposure with a simple structure.
It is another object of this invention to provide a charged beam exposure apparatus which can accomplish high-precision alignment and pattern exposure with a simple structure.
According to a first aspect of the invention, there is provided a charged beam exposing method comprising the steps of: applying a first voltage for accelerating a charged beam, emitted toward a sample by a charge-particle irradiating section, more than is accomplished by application of a reference exposure voltage to the sample and scanning an alignment mark formed in the sample with the charged beam to thereby acquire a position of the alignment mark; and applying the reference exposure voltage to the sample to carry out pattern exposure with the charged beam emitted from the charge-particle irradiating section.
According to a second aspect of the invention, there is provided A charged beam exposing method comprising: a first step of applying a voltage different from a reference exposure voltage to a sample; a second step of scanning the sample with a charged beam to thereby detect a position of an alignment mark formed in the sample; a third step of changing an applied voltage to the sample; a fourth step of detecting an amount of change in the position of the alignment mark caused by a change in the applied voltage; and a fifth step of adjusting a tilt of the sample based on the amount of change in the position of the alignment mark.
According to a third aspect of the invention, there is provided A charged beam exposing apparatus comprising: an charge-particle irradiating mechanism irradiating a charged beam to a sample; a voltage applying mechanism selectively applying a voltage for accelerating the charged beam to the sample at a time of alignment exposure; and a mark position detecting mechanism detecting charge particles from the sample generated by irradiation of the charged beam in the alignment exposure to thereby detect a position of an alignment mark formed in the sample.
According to this invention, an acceleration electric field is generated on the surface of a sample by applying a voltage to the sample and an alignment mark is detected using the accelerated charged beam. Even if a low-energy charged beam is used, therefore, the mark located deep from the sample surface can be detected. Further, even in which case where a conductive film is formed under the resist, high-precision detection of the alignment mark is possible which was difficult according to the conventional alignment method using a low-energy electron beam. Further, as charged beam exposure is carried out at a slower acceleration than mark detection, it brings about such advantages as suppression of the proximity effect by the slow-accelerated charged beam exposure apparatus and prevention of charge-up. Furthermore, alignment with the fast-accelerated beam and exposure with slow-accelerated beam can be switched from one to the other in response to a variation in the voltage applied to the sample. Therefore, this exposure apparatus does not require a separate fast-accelerated charge-particle irradiating mechanism and thus has a simple structure.
At the time of detecting the alignment mark, different acceleration voltages are applied to a sample, the deviation of the alignment mark on each applied voltage is detected and the mark position on a reference exposure voltage is calculated. Even if the sample is tilted to the objective lens in the charge-particle irradiating section, therefore, the mark position with the inclination corrected is acquired to ensure alignment exposure at higher precision.
In addition, alignment with the same energy from the charge-particle irradiating section as used in the actual pattern exposure becomes possible by exposing an alignment pattern on a sample with a fast-accelerated beam and scanning this alignment pattern with a slow-accelerated beam. When the sample is tilted to the objective lens, therefore, it is possible to avoid deviation of the beam caused by a difference between the acceleration voltage at the time of alignment and the one at the time of pattern exposure.
It is also possible to measure misalignment of an exposure pattern prior to development of the resist, by exposing a desired pattern after exposing an alignment pattern and obtaining a relative positional deviation between those alignment pattern and the desired pattern. This can guarantee selection of those samples which meet the allowable precision prior to development and can eliminate a wasteful process, thus leading to an improved productivity.
Further, a charged beam exposure apparatus, which detects the position of the mark and carries out pattern exposure with a voltage applied to a sample, can detect the inclination of the sample to the objective lens or a non-uniform electric field between the sample and the objective lens by detecting a change in mark position caused by varying the applied voltage to the sample.
Furthermore, after the change in mark position is detected, the inclination of the sample surface is readjusted so that a change in mark position, when the applied voltage to the sample is changed, falls within a tolerance range. Accordingly, it is possible to correct the tilt of the sample surface and a non-uniform electric field between the sample and the objective lens. As a result, the tilt of the sample surface is adjusted to ensure high-precision pattern exposure. Moreover, at the time of detecting the latent image of the resist or the image of the alignment mark, the accurate mark position can be acquired, thus ensuring high-precision alignment exposure.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.